Copending Application A discloses a transmission system wherein a microprocessor senses the rate of rotation of input and output shafts, and the positions of a gearshift lever and a clutch pedal to control the selection and energization of clutches in a transmission to thereby control the selection of the gear speed ratio between the input and output shafts. To improve clutch life or durability in such a system, my copending application B proposes a method of clutch control wherein the range of clutch pedal positions available for operator control is limited, and wherein energization of a selected clutch is under microprocessor control and independent of clutch pedal position when the clutch pedal is released above a position defined as the full pressure point. For clutch pedal positions below the full pressure point, the position of the clutch pedal is sensed and converted to a signal for controlling clutch pressure. When the clutch pedal is released to or above the full pressure point, the position is sensed and the microprocessor generates a ramp signal which linearly varies with time so that the clutch pressure is increased to its maximum value in a short interval of 1 to 2 seconds.
While this method does improve the durability of clutches by restricting the ability of the operator to slip a clutch, the ability of the clutch to dissipate heat may be exceeded under certain conditions. By way of example, the energy available to over-heat and damage a clutch during the ramp to full pressure at the full pressure point of the clutch pedal is a function of the square of the engine RPM. The worst case condition results from slipping the clutch at maximum engine speed and maximum available inching pressure until the clutch temperature stabilizes, and then releasing the clutch pedal and stalling the engine. The kinetic energy of the flywheel and other rotating parts is converted to heat in the clutch. The power into the clutch during a stall is much higher than the steady state power dissipation capacity of the clutch, so the temperature of the clutch plates rises. The amount of the temperature increase is determined primarily by the quantity of energy which the clutch must absorb and the thermal capacity (heat sink capability) of the clutch. The steady state temperature during inching plus the temperature rise during a stall should result in a low enough temperature to insure clutch durability, a condition not always obtainable with the above method.
When operating at a reduced engine speed, the stall energy is significantly reduced, which means that the steady state temperature during inching may be higher. Also, it becomes impractical to quickly stall the engine at speeds below 4-5 mph, thus higher steady state inching temperatures are practical as lower gears are selected. Taking advantage of these facts, the present invention provides a method of clutch control wherein the maximum inching pressure is controlled as a function of engine speed and gear selection, or pedal position depending upon which control will result in the lowest clutch pressure.